The present invention is directed to the compression of delta modulation coded sequences that represent successive scan lines of image data.
A delta modulation system is one in which each sample of an image is compared to a predicted value and the difference is encoded as one of only two binary levels. The Predicted value is formed using a linear combination of previously reconstructed signal values. The binary level is indicative of whether the difference is being approximated by a positive or a negative step. In other words, a delta encoder generates a binary one in the case of a negative-difference and a binary zero in the case of a positive-difference, or vice versa. In regions corresponding to a uniform gray level, scanned image data processed by a delta modulator will characteristically produce an alternating output of "101010 . . . " bits. The aforementioned bit sequence is valid for a 1-D predictor. For higher order predictors, e.g., 2-D predictors, the output bit string for a large uniform area will also be "10101 . . . ". Since the output bit strings of alternating ones and zeros occur frequently in delta modulated image data sequences, it is highly desirable to develop an efficient coding scheme which will compress these alternating strings.
It has been recognized that alternating strings of binary bits can be compressed using runlength encoding.
One patent of interest for its teaching of such a compression method is U.S. Pat. No. 4,005,411 entitled "Compression of Gray Scale Imagery to Less Than One Bit Per Picture Element" by T. H. Morrin, II. In the background section of that patent it is recognized that a constant scan input, when delta encoded, will output a string of "10101 . . . " bits. In the apparatus disclosed in that patent a predictor operates upon scan lines of data to predict the next bit. If the predictor is correct, a zero bit is recorded. If the predictor is not correct, a one is recorded. Generally, a long string of zeros will thus be generated. The long string can then be runlength encoded. The disclosed apparatus utilizes a logical mod 2 combining of the predictor's output signal with the delta encoded signal to derive an error image signal. The thus derived error image signal is runlength encoded.
In the aforementioned apparatus a decoding error will propagate through the reconstructed image due to the fact that the predictor makes subsequent use of previously encoded data.
Another patent of interest for its teaching is U.S. Pat. No. 4,136,363 entitled "Truncated Run Length Encoding" by A. Saran. The apparatus of that patent uses a current scan line and the previous scan line to generate a difference signal. The difference signal is formed as a mod 2 difference. In a manner similar to the preceding Morrin, II patent '411, errors can propagate through an image because previous lines of data are used to form a prediction for the current line. The inventor Morrin, II recognized this problem and provided as a solution that for every n lines of transmitted coded data that at least one line of uncoded data be transmitted so as to break the sequence of error propagation.
A delta encoder/decoder signal compression method and system which eliminates the propagation of error data while providing the benefits of signal compression utilizing runlength encoding is therefore desirable.